Optical Communications System

ABSTRACT

In an optical communications link, an optical system including: at least a first input port for delivering an optical signal travelling in the communications link, the optical signal including a plurality of wavelength channels, the channels being utilized for carrying optical information over an optical data link; a dispersive element for spatially separating the wavelength channels; an active optical-phase element; and a plurality of optical manipulation elements for directing the spatially separated channels between the dispersive element and the optical phase element wherein, the optical phase element independently modifies the phase of predetermined ones of the wavelength channel in a predetermined and decoupled manner for substantial compensation of signal degradation effects imparted to the wavelength channels by said communications link.

CROSS-REFERENCES TO RELATED APPLICATIONS

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STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

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FIELD OF THE INVENTION

The current invention relates to optical communication systems and inparticular to fiber optic communication systems. The opticalcommunications system of the present invention utilizes a property ofthe nonlinear Schrödinger equation whereby the individual signals in awavelength division multiplexing (WDM) system are decoupled from eachother and individually corrected for dispersive and nonlinear effectsaccumulated along each span of the optical network.

BACKGROUND OF THE INVENTION

The recent growth in the demand for broadband services has resulted in apressing need for increased capacity on existing communication channels.The increased bandwidth of optical fiber communication links are stilloften insufficient to cope with this demand without utilizing theability of these fibers to carry large numbers of individualcommunication channels, where each channel is identified by theparticular wavelength of the light. This technique is known as densewavelength division multiplexing (DWDM).

Linear optical fiber communication technologies are essentially based onthe same principles as radio frequency systems. Fiber communicationsystems, however, are fundamentally different, because they makepositive use of the inherent optical fiber property of nonlinearity.Rapid progress in nonlinear lightwave communications is stimulated byincreasing demand for telecommunications services. Practical andresearch interest is directed mostly toward two main goals: developmentof effective high capacity long-haul transmission systems and theupgrade of existing terrestrial fiber networks.

There are three major factors that cause optical signal degradation anddistortion in long-haul high bit-rates fiber communication systems:

Loss due to absorption in the fiber;

Group-velocity dispersion (GVD); and

Optical nonlinearity.

Signal power attenuation due to absorption can be compensated usingamplifiers, although recovery is not complete since amplifiedspontaneous emission noise is added to the signal and degrades thesignal-to-noise ratio. Revolutionary developments in nonlinear lightwavecommunications have been triggered by the development and deployment ofoptical amplifiers providing periodic amplification of optical signals.The portion of the optical link between each amplifier is commonly knownas a span, where each successive amplifier at a node position amplifiesthe optical signals that have degraded by propagation through theprevious span.

Until the invention of the erbium-doped fiber amplifier (EDFA), opticalsignals were regenerated electronically to overcome the attenuation inthe silica fiber. Electronic regenerators have two important drawbacks:they are expensive and they limit system performance, because eachregenerator can operate at only one predetermined bit-rate, in one datamodulation format and at one operating wavelength.

Because the EDFA has many important advantages (such as large bandwidth,high gain, simplicity and others) over optoelectronic regenerators, theyquickly became the amplifier of choice in communication systems. As aresult, fiber loss is no longer a major limitation in optical fibertransmission and the performance of optical amplifier systems is thenlimited by CD and nonlinearity. Note that whereas a regeneratorre-creates a perfect digital output signal, the fiber amplifier useswhatever it receives. Therefore, dispersive pulse broadening and otherdegrading effects are accumulated along a fiber line.

There are two principal approaches to overcome these limitations: in thefirst instance, both the chromatic dispersion (CD) and nonlinearity areconsidered to be detrimental factors, however, due to short fiberlengths and small optical intensities in the fiber, the nonlineareffects are only small and therefore are ignored. This is known as alinear approach. In the second approach, the nonlinear and dispersiveeffects are recognized under certain conditions as being reciprocaleffects that can be counterbalanced by appropriate design of the linkarchitecture. Such systems are called nonlinear since the nonlineareffects that are detrimental in the linear systems are used to improvetransmission characteristics of the optical communication system.

As the demands on the capacity of the communications links increasesfurther, the modulating frequency of the optical signals must increase.Current standard systems operate at data transmission rates of 10 Gbitsper second per channel so that, in a DWDM network containing 50channels, the total transmission capacity of the link is 500Gbits/second. The next evolution in communications links requires thisto increase and the next goal is to have reliable links operating at 40Gbits/s per channel. At this data rate, electronic components used inrepeaters and at the receiving end of the link struggle to keep pacewith the amount of data being transmitted. Thus the demand is to be ableto transmit the information through the link in wholly optical form,without any conversion to an electronic signal. This will certainly berequired for the further generations of optical communications linkswhere data rates of 100 Gbits/s per channel and greater are predicted.

A light pulse is an electromagnetic wave packet built from a continuumof elementary optical carriers oscillating at different frequencies. Inother words, any optical wave-packet contains a range of frequencycomponents. Since any optical fiber is a dispersive medium, each ofthese spectral components travel at different group velocities, causingthe pulse energy to spread over time as the pulse propagates through themedium. Fiber GVD is measured either in units of picoseconds squared perkilometer (ps²/km) or picoseconds per kilometer per nanometer(ps/km.nm). Roughly speaking, a pulse with the bandwidth 1 nm spreads bycorresponding number of ps over 1 km. Dispersion can be positive wherelow frequencies travel at a higher speed than high frequencies ornegative where high frequencies propagate at a higher speed than lowfrequencies. The dispersion of standard single mode fiber (SMF) ispositive (also called normal) for wavelengths shorter than about 1300 nmand negative (anomalous) for wavelengths longer than 1300 nm. SMF hasdispersion of about 20 ps²/km at wavelength 1550 nm. Correspondingdispersive spreading of a 10 ps pulse in SMF after 125 km is about 50 psor, in other words, 5 times its original width. Such a large spreadingcan lead to overlapping in the time domain of neighboring bits andconsequently to degradation of the information signal.

Linear signal distortion caused by the GVD in fiber transmission systemscan be almost suppressed by the dispersion compensation or dispersionmapping technique. Optimization of the system performance in the case ofa linear transmission requires minimization of the CD of the opticalcommunications link. This can be achieved by operating close to the zerodispersion point or/and additional compensation of the accumulateddispersion. The idea to use a compensating fiber to overcome dispersionof the transmission fiber was proposed in 1980. In the low power linearregime where the response of the fiber is linear, compensation ofdispersion aims to prevent dispersive broadening of the signal in thetransmission fiber by the compression in the compensating fiber.

In linear systems dispersive broadening can largely be eliminated bydispersion compensation. However, the nonlinear effects can still be theprimary reason for signal degradation especially in long-haultransmission systems. The response of the optical medium is notexclusively linear. The fiber refractive index instantaneously increasesby an amount proportional to the optical power. This phenomenon is knownas the optical Kerr effect. Modulation of the optical power leads to thecorresponding modulation of the index. For instance, a high power lightpulse increases the refraction index with corresponding change of thephase of the propagating pulse. This effect is known as self-phasemodulation (SPM).

SPM involves an interaction of an optical pulse with itself. SPM of anoptical pulse does not cause any degradation of other bit-pulses ofoptical signals of different frequencies propagating through the linkand so does not significantly contribute to the interchannel cross-talkdegradation. When the dispersion map of an optical link is designed, thedispersive effects of the individual components that make up each spanin the map are considered and selected to optimize the SPM to acceptablelevels across the whole link. SPM can even be beneficial when thewavelength of the signal pulses falls in the anomalous region of theoptical fiber characteristics as it can partially compensate effects ofCD in the pulse by delaying the ‘fast’ spectral components relative the‘slow’ components.

The dispersion compensation technique has been used relativelysuccessfully both in long-haul communication systems and in the existingterrestrial optical links, most of which are based on standardtelecommunication fiber with large dispersion in the optical windowaround 1550 nm. The basic optical-pulse equalizing system consists of atransmission fiber, which is typically standard SMF already existing inthe installed link, and one or more lengths of equalizing fiberpossessing a large dispersion coefficient of opposite sign known asdispersion compensating fiber (DCF).

Prior art dispersion management schemes such as those disclosed in U.S.Pat. Nos. 6,832,051 to Lu et al and 6,417,945 to Ishikawa et al, whilebeing effective for single channel fiber communication systems, have atleast one shortcoming with regard to multi-channel systems.Specifically, complete correction of dispersion in all channels at theend of the system is not easily accomplished, primarily because thedispersion slope in the compensating fibers typically cannot meet thetwo requirements of being both high in magnitude and negative in sign.Thus, fibers with high negative dispersion and high negative slope aredifficult to manufacture and therefore expensive. Small variations infibers with these properties typically lead to large changes of otherproperties of the fiber, and hence are typically not reliablymanufacturable. Also, there is a large installed base of SMF fiber, andeven if DCF were easier to manufacture, replacing of the existingoutside cable plant would be very costly.

A further disadvantage of using dispersion-compensated fiber (DCF)includes the added loss associated with the splicing to the initialfiber length and the increased fiber span. Thus, the amplifier stage ineach span of the communications link must also compensate for thisadditional loss. Additionally, the nonlinear effects may degrade thesignal over the long length of the fiber if the signal is of sufficientintensity.

A dispersion management scheme that has been applied to a multiplechannel transmission system is disclosed in U.S. Pat. No. 6,659,614 toKatayama. This patent discloses separating the individual wavelengthchannels in the signal and directing them onto a single deformablemirror. The mirror is then deformed into a substantially parabolic shapeto correct for large-scale optical dispersion across the whole opticaltransmission window of the communications link. This patent, however,does not teach individual control over each channel independently. Assuch the ability to control different transmission impairments islimited.

US Patent Application 2003/0170939 to Moon et al discloses a chromaticdispersion compensation device based on a micromirror array. Moonprovides a system whereby each of the dispersed wavelength channels isincident on a plurality of micromirror “pixels” which are switchedbetween one of two positions to delay part of the light incident on thearray by a predetermined amount and partially compensate for chromaticdispersion in a wavelength channel through providing a number ofquantized phase levels.

Disclosures by Katayama or Moon et al. both rely on mechanicaladjustments of mirrors which is disadvantageous in a high reliabilityTelecommunications environment and furthermore neither teach theadvantageous use of smooth adjustment of phase within a channel to varygroup delay in combination with control of the relative group delay orrelative phase between wavelength channels or the simultaneouscompensation of a variety sources of optical signal degeneration in anoptical communications link.

Other techniques used for dispersion compensation at each node in thedispersion map include:

-   -   Multiple-Cavity Etalons, which includes Gires-Tournois        Interferometers (GTIs), such as those disclosed in U.S. Pat. No.        6,768,874, 6,748,140 or 6,654,564, and by D. Moss et al,        Presentation TuD1 entitled “Tunable Dispersion Compensation at        10 Gbit/s and 40 Gbit/s Using Multicavity All-pass Etalons”,        Conference on Optical Fiber Communication (OFC) 2003, Atlanta        Ga., USA; or    -   Chirped Fiber Bragg Gratings (CBFGs) such as those disclosed in        U.S. Pat. No. 6,847,763 or 6,807,340.

The GTI is a bulk optics element that can be configured to operate ineither reflective or transmissive mode and compensates for dispersionthrough wave interference mechanisms. Since it is a bulk optic element,it can be configured to provide a high compensating dispersion through arelatively short propagation distance. Additionally, it can beconstructed such that the dispersion at a given center wavelength istunable. The most practical mode in which a GTI is used is in reflectionwhere the back face of the resonator is 100% reflective, however thesesuffer from high insertion losses due to the need to isolate the backpropagating signal. This can be partially compensated by the use ofoptical circulators, however, GTIs suffer from the fundamentaldisadvantage that the effective bandwidth decreases as the dispersion isincreased.

In contrast, the CFBG can be designed to simultaneously have a highdispersion and large bandwidth. It does, however, still operate in thereflective mode and so still suffers from the insertion losses and theneed for optical circulators within the span. Fiber Bragg gratings canalso induce dispersion ripple, which leads to undesirable distortion ofthe optical signals.

All these existing dispersion compensating techniques, however, possessa common problem. Since the amount of dispersion that each opticalchannel experiences depend on the frequency of the channel, it isextremely difficult to completely return all the channels to a commondispersion value at each node. This effect is depicted in FIG. 1 whichshows the (exaggerated) GVD over two spans in a dispersion managedoptical communications link. A plurality of optical channels are shownω₁ to ω₄ (where ω₁<ω₄). As the optical signals propagate through the SMF1 of Span 1, the signals of higher frequency experience a greater GVD.When the signals reach the length of DCF 2, the highly negativedispersion partially reduces the GVD. The problem lies with thedifficulty in correcting for the GVD across all the optical channels andso the signals enter the next span of the link with a GVD mismatchindicated by the 3. As the signals pass through multiple spans of thelink, the GVD mismatch increases as can be seen at the end 4 of thelength of DCF fiber 5.

The total GVD mismatch accumulated through all the spans of the linkneed to be compensated at the final receiver stage of the system. Atpresent, this is achieved by separating each of the DWDM channels on thelink and feeding them into separate electronic dispersion compensators.Again, these electronic systems are adequate for current data rates of10 Gbits/s, but struggle at increased data rates of 40 Gbits/s andupward. Thus, the need for all optical dispersion and GVD compensatorsfor further advances in optical communications systems is paramount.

As has been discussed, the detrimental effects of the optical fiber linkin current systems are managed by modeling of the whole opticalcommunications link and inserting lengths of fiber into the link atregular intervals to amplify the pulse to counter the effects of opticalloss in the fiber in the time domain, and optical dispersion in thefrequency domain. This is a valid method, provided that the opticalsignals propagating through the link experience all the spans asdesigned.

In reconfigurable networks, where optical signals can be added anddropped to the link at any number of stages along its length byreconfigurable add/drop multiplexers (ROADMs), the dispersion map isless valid. The optical signals have not propagated through each stageto get the benefit of the carefully designed dispersion and GVDproperties of the link as a whole. Extra care must be given to thesesignals to determine their properties once they have been dropped fromthe link and compensated to avoid loss of the information they contain.

CO-PENDING APPLICATION

Various methods, systems and apparatus relating to the present inventionare disclosed in the co-pending U.S. application Ser. No. 10/706,901filed on 12 Nov. 2003 by the applicant or assignee of the presentinvention. The disclosures of this co-pending application areincorporated herein by cross-reference.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved opticalcommunications system.

In accordance with a first aspect of the invention there is provided anoptical system including: an input port for accepting an optical signal,the optical signal including a plurality of wavelength channels, eachwavelength channel having a predetermined spectral width; a plurality ofindividually addressable phase control elements for discretely modifyingthe phase of each of the wavelength channels independently, where themodification is performed at a scale less than that of the spectralwidth of each channel to provide a plurality of phase-modifiedwavelength channels; and at least one output port for distributingselected ones of the phase-modified wavelength channels to an opticalcommunications link.

Preferably, the phase control element further discretely andsubstantially independently modifies the amplitude of selected ones ofthe wavelength channels in accordance with requirements

In accordance with a second aspect of the invention there is provided anoptical communication system including a plurality of optical systems,where each of the optical systems is distributed at discrete locationsin the communications system.

In accordance with a third aspect of the invention there is provided anoptical compensation system including: an optical input port forreceiving an optical signal, the optical signal having a plurality ofoptical channels; an optical dispersion element for spatially dispersingthe optical channels; an optical compensation element for individuallymodifying the relative phase of each optical channel to produce a seriesof spatially dispersed phase modified optical channels; and an opticalcombining element for combining the spatially dispersed phase modifiedoptical channels to produce an optical output signal.

Preferably, the optical dispersion element and the optical combiningelement are both selected from the group consisting of an opticalgrating and a grism.

More preferably, the optical compensation element includes a series ofspatially dispersed relative phase manipulation elements for eachspatially dispersed optical channel, where the phase manipulationelements are capable of independently manipulating spatially distinctportions of a spatially dispersed optical channel.

In accordance with a fourth aspect of the invention there is provided amethod of selectively compensating a plurality of optical channelscontained in an optical signal, the optical signal propagating in anoptical communications link including the steps of:

-   -   a) receiving the optical signal on an input port of an optical        wavelength processor;    -   b) spatially dispersing he optical channels in the optical        signal along a dispersion axis;    -   c) directing each of the spatially dispersed optical channel        onto a predetermined region of an optical phase manipulation        device, the optical phase manipulation device including a        plurality of independently addressable pixel elements;    -   d) setting each of the pixel elements to one of a plurality of        predetermined levels such that the phase of the light incident        on each pixel element is modified by a predetermined amount to        compensate for degenerative effects accumulated in the optical        communications link to provide a plurality of compensated        optical channels;    -   e) recombining the compensated optical channels to provide an        optical output signal; and    -   f) outputting the optical output signal on an output port.

Preferably, the relative group velocity dispersion mismatch betweenselected adjacent pairs of the optical channels is controlled. Morepreferably, the relative phase of one of the optical channels in eachpair of channels receives an additional phase shift. More preferablystill, the group velocity mismatch is adjusted to be substantially zeroand the phase shift is substantially pi radians.

The pixel elements are preferably addressed by a group delay functionthat is substantially continuous within a wavelength channel andprovides the ability to control the pixel elements substantiallydiscontinuously between adjacent channels. Preferably, the groupvelocity dispersion mismatch and phase between selected pairs of nextadjacent optical channels are controlled. The degenerative effectspreferably include one or more of chromatic dispersion, group velocitydelay, optical nonlinearity, cross-phase modulation and self-phasemodulation.

Preferably, the spatially dispersed optical channels are modified on ascale less than the spectral width of the bandwidth of each channel.More preferably, the pixel elements are arranged spatially into columns,the columns of pixel elements being angularly oriented with respect tothe dispersion axis to substantially reduce phase quantization effectsof the compensation imparted to each optical channel. More preferablystill, the optical phase manipulation device modifies selected ones ofhe spatially dispersed optical channels along an axis orthogonal to thedispersion axis such that the direction of propagation is modified toprovide a plurality of optical output signals. Still more preferably,each of the optical output signals is output on a predetermined one of aplurality of output ports.

In accordance with a fifth aspect of the invention there is provided inan optical transmission system including: an optical transmitter fortransmitting a plurality of independent wavelength channels; at leastfirst and second spans, each span respectively including: at least afirst length of transmission optical fibre characterized by a positivedispersion coefficient; and at least a first length ofdispersion-compensating optical fibre disposed after the length oftransmission fibre and characterized by a negative dispersioncoefficient for substantially compensating a large proportion of thegroup velocity delay accumulated by each wavelength channel in the firstspan; a first node module disposed after the first span including: anoptical amplifying module; and a phase manipulation module for:

-   -   a. substantially compensating for dispersion of each wavelength        channel accumulated in the first span;    -   b. substantially compensating for the remaining accumulated        group delay in selected pairs of first and second wavelength        channels; and    -   c. modifying the phase of the second wavelength channel in the        selected pair such that the cross talk between the selected pair        of channels generated in the first link is at least partially        compensated in the second link.

The system preferably further includes a second node module disposedafter the second span including; an optical amplifying module; and aphase manipulation module for substantially compensating for theremaining accumulated group delay in selected pairs of third and fourthwavelength channels and for modifying the relative phase of thewavelength channels.

Preferably the system also includes: a plurality of sequentiallydisposed pairs of the first and second spans, each interspersed betweenone of a plurality of respective first and second node modules; and areceiver module for receiving the transmitted wavelength channels.

In accordance with a sixth aspect of the invention there is provided anoptical transmission system including: an optical transmitter fortransmitting a plurality of independent wavelength channels; at leastfirst and second lengths of transmission optical fibre characterized bya first predetermined dispersion coefficient; at least first and secondlengths of dispersion-compensating optical fibre characterized by asecond predetermined dispersion coefficient for substantiallycompensating a large proportion of the group velocity delay accumulatedby each wavelength channel in respective lengths of transmission opticalfibre, the first length of dispersion compensating fibre disposed afterthe first length of transmission fibre and the second length ofdispersion compensating fibre disposed after the second length oftransmission fibre; a first node module interposed between the firstlength of dispersion compensating fibre and the second length oftransmission fibre including: a phase manipulation module configured to:substantially compensate the chromatic dispersion accumulated by each ofthe wavelength channels in the first length of transmission fiber;substantially compensate for the remaining accumulated group delay inselected pairs of first and second wavelength channels; modifying thephase of the second wavelength channel in the selected pair such thatthe modified phase of the second channel is substantially out of phasewith the first wavelength channel in the selected pair; and a secondnode module disposed after the second length of dispersion compensatingfibre including; an optical amplifying module; and a phase manipulationmodule for substantially compensating for the remaining accumulatedgroup delay in selected pairs of third and fourth wavelength channelsand for modifying the phase of the wavelength channels such that themodified phase of all the wavelength channels are substantiallyin-phase.

Preferably, the phase manipulation module modifies the phase of thewavelength channels in a decoupled manner such that nonlinear opticaleffects accumulated by any of the wavelength channels in the firstlength of transmission fibre are reversed in the second length oftransmission fibre. More preferably, the first and second predetermineddispersion coefficients are of opposite sign.

In accordance with a seventh aspect of the invention there is providedin an optical communications link, an optical system including: at leasta first input port for delivering an optical signal travelling in thecommunications link, the optical signal including a plurality ofdiscrete wavelength channels, each channel for carrying opticalinformation over an optical data link; a dispersive element forseparating each channel from the optical signal; an active optical-phaseelement; and a plurality of optical manipulation elements for directingthe separated channels between the dispersive element and the opticalphase element wherein, the optical phase element independently modifiesthe phase of each of wavelength channel in a discrete and decoupledmanner for substantial compensation of dispersive and nonlinearcharacteristics imparted to the wavelength channels by thecommunications link.

Preferably, the optical phase element independently modifies the groupdelay of the wavelength channel. More preferably, the optical phaseelement independently modifies the dispersion of the wavelength channel.Still more preferably, the optical phase element independently modifiesthe amplitude of the wavelength channel.

The system preferably includes further optical manipulation elements fordirecting the modified wavelength channels to a further dispersionelement. More preferably, the further dispersion element combinesselected wavelength channels to form an optical output signal, theoptical output signal being directed to one of a plurality of opticaloutput ports.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description of preferredembodiments of the invention, taken in combination with the appendeddrawings in which:

FIG. 1 is a graph depicting a prior art dispersion map of the groupvelocity delay of a plurality of optical channels in a DWDMtelecommunications link.

FIG. 2 shows the optical intensity of a plurality of wavelength channelsincident on discrete regions of a wavelength processing device whereeach individual channel receives a predetermined compensating phaseprofile across the spectral width of the channel.

FIG. 3 is a schematic of the components in a typical telecommunicationlink.

FIG. 4 shows a detail of a node of the telecommunications link of FIG. 3including an embodiment of the present invention.

FIG. 5 shows a detail of a node of the telecommunications link of FIG. 3including a second embodiment of the present invention.

FIG. 6 is a schematic diagram of the optical processing device of thepreferred embodiment of the invention.

FIG. 7 is a detail of a region of the phased-matrix processor of thepreferred embodiment and an arbitrary phase profile set up in the pixelsof the region to modify the phase of an incoming wavelength channel.

FIG. 8 is a schematic top view of the phased-matrix processor showingthe pixels of the device and the segmentation of the device intoregions, each region for modifying a spatially separated wavelengthchannel. Also displayed in this figure are the typical intensityprofiles of the wavelength channels in each orthogonal plane in thepreferred embodiment of the system.

FIG. 9 is a graph of arbitrary phase profiles that can be set up in thepixels of the phased-matrix processor across two regions of theprocessor corresponding to two adjacent wavelength channels.

FIG. 10 is a three-dimensional representation of an arbitrary phaseprofile set up on a liquid crystal optical wavelength processor for asingle wavelength channel.

FIG. 11 is a section of the phase profile in the dispersion axis takenon line A-A of FIG. 10.

FIG. 12 is a section of the phase profile taken on line B-B of FIG. 10.

FIG. 13 is a graph showing the nonlinear FWM interaction strengthbetween two in-phase pulses of different frequency as they propagatethrough a length of standard SMF.

FIG. 14 is a graph showing the nonlinear FWM interaction strengthbetween two out-of-phase pulses of different frequency as they propagatethrough a length of standard SMF.

FIG. 15 is a graph of the intensity of the FWM product between twopulses of different frequency as they propagate through two spans of atelecommunication link in accordance with preferred embodiments of theinvention.

FIG. 16 is graph depicting a dispersion map of the group velocity delayof a plurality of optical channels in a DWDM telecommunications linkwith preferred embodiments of the invention.

FIG. 17 is an overview of the optical compensation method of thepreferred embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments provide for an improved optical communicationssystem which may have the following advantages:

-   -   Compensation of dispersion effects on a channel-by-channel        basis.    -   Mitigation of nonlinear four-wave mixing effects.    -   Be reconfigurable to dynamically adjust each individual        wavelength channel independently to account for mismatches in        current dispersion compensation techniques or thermally induced        variations.    -   Be able to accommodate optical signals that have been added to        the link at various add/drop locations and compensate for        dispersive and nonlinear effects that have not experienced the        full length of the dispersion-managed system.    -   Independent control over both amplitude and phase of DWDM        signals on a channel-by-channel basis.

The preferred embodiments of the present invention disclose methods ofrealizing the characteristics of an improved optical communicationssystem by providing means of controlling the relative phase of receivedoptical signals both on the scale of the individual wavelength channels,and also independently controlling the relative phase on a scale lessthan that of the spectral width of each of the individual channels.Advantageously discrete amplitude control of each channel can also beprovided for power level balancing between optical channels to suppressthe 4 wave mixing components and to optimize the signal to noise ratioof all channels. The preferred embodiments provide an electronicallycontrollable method of phase and amplitude control over the individualoptical wavelength channels without the need to first convert eachoptical channel into an electronic signal. This unique property enablesunparalleled scope for increased communications bandwidth over opticalcommunication links to deal with the ever-increasing demand oncommunications services. An overview of the method is depicted in FIG.17.

Overview

In the most general case, signal transformation along the fibercommunication link is caused by a combined action of dissipation andamplification, dispersion and nonlinearity and cannot be described in asimple way. The basic mathematical model that forms the theoreticalbackground for fiber optic communications is the nonlinear Schrödingerequation (NLSE). Optimization of the optical transmission systemparameters is a crucial task for the design of fiber links. Usually,time-consuming numerical simulations involving the NLSE are required tofind optimal operating regimes and optimal system parameters.Comprehensive investigation of stable regimes and their tolerance inmulti-dimensional parameter space is limited by the computational timerequired for optimizations.

Propagation of optical pulses through an optical fiber including bothdispersive and nonlinear effects can be described using the nonlinearSchrödinger equation (NLSE)

$\begin{matrix}{{{\frac{\partial E}{\partial z}} + {\frac{1}{2}{\beta (z)}\frac{\partial^{2}e}{\partial t^{2}}} + {{\sigma (z)}{E}^{2}E}} = 0} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where the first order GVD coefficient β(z) in ps²/km is expressed interms of the dispersion D (in ps/km/nm) as

β(z)=−(λ₀ ² D/2πc)  (Equation 2)

The term |E²| is the optical power P in watts and gives rise to thenonlinear optical Kerr effect where the nonlinear coefficient is givenby

σ(z)=2πn ₂/λ_(o) A _(eff)  (Equation 3)

In these equations E is the complex wavefunction of an optical signalpropagating in the communications link, i=√{square root over (−1)},λ_(o), is the carrier wavelength of the optical signal, n₂ is thenonlinear refractive index of the fiber, c is the free space velocity oflight, and A_(eff) is the effective mode area of the fiber core.

One analytical solution to the NLSE is known as an optical soliton thathas been shown to be feasible for long-haul optical communications. Anoptical soliton utilizes the opposing effects of CD and SPM within theoptical fiber to enable a pulse to be propagated for many thousands ofkilometers before requiring amplification. Soliton communicationsystems, however, have experienced a slow adoption rate due to the largecost required to convert currently installed communications equipment tobe able to handle soliton transmissions. Therefore, optimization offiber optic communications requires modeling of optical pulses with amore general structure, for instance, that of a standard traveling wavesolution such as can be described as

E=E _(o)exp[i(ωt−kz+φ)]  (Equation 4)

with a frequency ω, wavevector k, and initial phase φ. For these casesthe NSLE must be solved numerically for the communications link. Onetechnique, known as the split-step Fourier method considers theindividual actions of SPM and GVD, computing each as independent actionsthat accumulate as the pulse propagates through discrete segments of theoptical fiber.

This numerical technique requires that the optical signal be transformedbetween the time- and frequency domains at each iteration to be able tocompute the different effects. In this way, the phase information isinextricably linked to that of the GVD and the dispersion of the opticalsignal. As well as the multiple Fourier transforms required by thealgorithm, the numerical technique also becomes excessive in terms ofcomputation time due to:

-   -   The long distances of a communications link that must be        completely characterized from transmitter to final receiver, and    -   The small fiber length segments that must be used in order to        gain an accurate representation of the optical signal as it        propagates through the fiber.

Whilst the preferred embodiment does not eliminate the need for modelingof the communications link, it can reduce some of the tolerances of themodels due to the reconfigurable nature of the compensation at eachnode. This is also useful as the optical properties change over time asthis can be compensated for electronically from a central locationsimply by adjusting the level of compensation provided by the device.When a system is designed for use at one channel bit rate (e.g. 10Gbit/s), the same system may be able to be used at a higher channel bitrate (e.g. 40 Gbit/s) by exploiting the increased margin that isprovided in the preferred embodiment.

Group velocity delay (GVD) is defined as the change in phase φ of anoptical signal as a function of frequency ω.

$\begin{matrix}{T = \left. \frac{\Delta\varphi}{\Delta \; \omega}\Rightarrow\frac{\varphi}{\omega} \right.} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

Dispersion of optical signals is defined as the change of the GVD as afunction of frequency:

$\begin{matrix}{D = {\frac{\Delta \; T}{\Delta \; \omega} = \left. {\frac{\Delta}{\Delta \; \omega}\frac{\Delta \; \varphi}{\Delta \; \omega}}\Rightarrow\frac{^{2}\varphi}{\omega^{2}} \right.}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

The dispersion can be positive or negative depending on the wavelengthof the optical signals and the properties of the optical fiber. Thus, ina dispersion-managed communication link at any point, if the phase of anoptical pulse is known then the GVD and dispersion of the optical signalcan be determined for any particular wavelength channel of knownfrequency, or vice-versa, by using the NLSE. It is this property thatdefeats the dynamic compensation of the channels in the communicationslink at any point without first converting the optical signal to anelectronic one and suffering the bandwidth restrictions that entails.

Compensation of Dispersion and GVD Effects

If each of the individual wavelength channels could be de-coupled fromall the others in the optical domain the phase of each of the signalscould then be modified independently. This can be realized using adispersive element such as an optical grating or prism and has beendemonstrated for beam steering applications by modification of the phasefront of the channel perpendicularly to the dispersion axis.

An improvement on this system is the ability to modify the phase of thedispersed optical wavelength channel in the dispersion axis on a scaleless than that of the frequency bandwidth of the channel itself. Sincethe phased-matrix wavelength processor is constructed from discreteindividually controllable pixels this gives discrete electronic controlover the light. The control can be further improved by angularlyoffsetting the phased-matrix processor slightly with respect to thedispersion axis. This gives the added advantage of smoothing the phaseprofile that the light incident on the pixels experiences. The angularoffset needs only be very slight to achieve a significant smoothingeffect of the discrete nature of the phase profile experienced by thelight, typically on the order of the pixel width of the wavelengthprocessor divided by half the width of the optical beam in thedispersion axis. In the current embodiments of the system this equatesto a sub-degree offset approximately 0.3°.

FIG. 2 shows an example phase map as a function of frequency incrementsfor a number of optical channels in a DWDM communications system wherethe phase of each of the channels incident on an optical phased-matrixprocessor is modified in a dynamic fashion. Channel A (10) experiences auniform phase shift across the bandwidth of the channel by an arbitraryamount by the phased-matrix processor. When Channel A leaves the device,it still retains the CD that it has accumulated in traveling through theoptical fiber span before reaching the processor. In contrast, theadjacent Channel B (11) experiences a parabolic phase shift across thebandwidth of the channel to compensate for the effect of CD in theprevious span. The parabolic nature of the modulation across the channelin the frequency domain is selected based upon the quadratic nature ofthe dispersion as can be seen on inspection of Equation 6. Channel C(12) experiences the same amount of CD compensation as Channel B, butalso experiences an additional linear phase shift. Channels D and E (13and 14 respectively) experience respectively a negative and positivegroup velocity delay across the channel in analogy with Equation 5 butsince there is no quadratic component to the induced phase profile, noCD compensation occurs on these Channels. Channel F (15) experiencesboth a positive group delay and CD compensation.

As can be seen from FIG. 2, each of the optical channels within thecommunication link has been de-coupled from the other channels whichtherefore allow the phase as a function of frequency across the channelto be modified independently. This is achieved by first dispersing thechannels and directing them to be incident on an electronicallycontrollable phased-matrix processor.

A typical optical communications system is shown schematically in FIG.3. A plurality of unique DWDM optical signals is generated by atransmitter 20 into standard SMF 21 (generally leaf fiber with adispersion coefficient of between 16-20 ps/nm.km). A length of DCF 22 isconnected to the SMF after a predetermined length to compensate for themajority of the accumulated GVD across the channels. The combination ofthe SMF and DCF are generally considered as a single span. After thelength of DCF the signals enter a node 23 where the signals aretypically only amplified by a length of EDFA 24. Some opticalcommunications links convert the optical signals to electronic signalswithin the node for further signal processing as required. The signalsare then re-transmitted into a second length of SMF and theconfiguration repeats until the signals are collected at a receiver 32.The signals are then typically converted to electronic signals forcorrection of residual GVD, dispersion and nonlinear effects and routedto either to their final destination or reconverted to optical signalsfor re-transmission on a subsequent communications link.

In the preferred embodiment of the current invention, a further opticalwavelength processing device 28 as shown in FIG. 4 is inserted into thenode. In further embodiments of the invention as shown in FIG. 5, theoptical processing device 29 can also include capabilities of areconfigurable optical add-drop multiplexer (ROADM) similar to thatdisclosed in the related U.S. patent application Ser. No. 10/868,521 toFrisken. In this embodiment, the optical processing device includes aplurality of add ports 30 and a plurality of drop ports 31. Particularwavelength channels can be extracted from the primary transmission linkto be re-routed via the drop ports and other wavelength channels can beadded to the communications link via the add ports.

A schematic diagram of the optical wavelength processing device is shownin FIG. 6. The DWDM signals enter the device on an optical fiber 35 andenter a series of conditioning optics 40. In the preferred embodiment,the conditioning optics includes a lens aligned with the input port 35to decrease the numerical aperture (NA) of the optical fiber. Thisrelaxes the requirements on the optical quality of the subsequentoptical elements. In the ROADM embodiment, the single lens becomes anarray of lenses each aligned with a corresponding input port, either themain input port 35 (also known as the express port) or one of the addports 30.

The conditioning optics 40 also includes polarization manipulation andequalization elements to substantially place all the light emerging fromone of the inputs in a single polarization orientation for efficientprocessing of the signals by the subsequent optical elements that areusually polarization sensitive. The polarization manipulation andequalization elements can include composite λ/2 waveplates and orbirefringent wedge (BRW) elements such as those disclosed in U.S. patentapplication Ser. No. 10/868,521 to Frisken.

The optical signals are then transmitted through free space to anoptical diffraction element 41 such as an optical grating, prism orgrism available from for example Newport, Spectra Physics or a multitudeof alternate optics suppliers, where the propagation direction of theindividual wavelength channels are slightly angularly separated withrespect to each other such that after a predetermined distance thesignals are each wholly spatially separated. Each of the optical signalsis characterized by a pre-defined spectral width Δω that is set for thepublic networks by the International Telecommunications Union (ITU) withchannel spacings ranging between 200 GHz and 12.5 GHz.

The spatially separated optical signals are then incident onto aphased-matrix optical processor 42 that is placed at the predetermineddistance away from the diffraction element 41. The optical processor isa two-dimensional array of individually addressable pixel elements 44that operate on the phase of the incident optical signal. An example ofa suitable processing device is that of a liquid crystal array commonlyused for display and projection applications and available from avariety of sources such as MicroDisplay of San Pablo Calif., USA.

The individual wavelength channels are redirected to a second opticaldiffraction element 45 to be recombined. A second series of opticalconditioning element 46 focuses the recombined DWDM channels into anoptical output fiber port 47, which, in the preferred embodiments of theoptical communications system, is the SMF stage (25 of FIG. 3) of thesubsequent span in the communications link.

FIG. 6 of the phased-matrix optical processor is depicted in atransmissive mode. The phased-matrix optical processor can also beoperated in a reflective mode where, in this case the opticaldiffraction elements 41 and 45 are the same element, and the opticalconditioning elements 40 and 46 are also the same elements. Thisreflective configuration is preferred since it reduces the complexityand improves the manufacturability of the optical processing device.

The phased-matrix optical processor is divided into a plurality ofregions in the same plane as the optical diffraction element such thateach of the wavelength channels is incident on one of the regions asshown in FIG. 7, in this case the wavelength channel ω₁ is shownstriking the region 43 with an intensity profile 48. FIG. 8 showsschematically a phased-matrix optical processor 60 that has individuallyaddressable pixels 44 on its surface. Each of the wavelength channels isincident on a region 43 of the phased-matrix optical processor with anintensity distribution 63 in the dispersion axis (axis indicated byarrow 64). In the preferred embodiment, the regions each includeapproximately 10 to 12 pixels in the dispersion axis and approximately500 pixels in the orthogonal axis. In other embodiments the number ofpixels in either axis can be increased to give greater resolution andhence finer control of the phase of the channel as required. Theintensity distribution 65 of each of the channels in the orthogonal axisis also shown. The pixels 44 are each driven to one of a number oflevels to modify the phase of the light that is incident on that pixel.Each of the regions includes a plurality of pixels 44 in the diffractionplane such that the phase of the individual channels can be modified asa function of the frequency of the light. Each of the individual pixelswithin the spectral width of the individual channel, i.e. occurring withthe corresponding region 43, can be electronically controlled to give adesired amount of phase shift to the optical signal.

The profile of the phase modulation across the channel in the frequencyaxis can be configured to compensate for propagation effects of thepreceding optical fiber span such as CD and GVD mismatch betweenchannels. Modification of the pixel levels along the axis orthogonal tothe diffraction axis can be used for switching applications for examplein an ROADM where the channels can be individually directed to anotheroptical fiber, as in a drop port. An example of this functionality isseen in U.S. patent application Ser. Nos. 10/706,901 and 10/868,521 toFrisken.

FIG. 9 shows a graph of driving levels of the individual pixels 44 ofthe optical phased-matrix processor across two wavelength channels inthe frequency axis (pixels 66 of FIG. 8). In the first region 67 of thephased-matrix optical processor the pixels are configured along thefrequency axis (also known as the dispersion axis of the processor) toprovide a controlled amount of CD compensation to the optical wavelengthchannel (Channel A) incident on this region. The parabolic nature of thephase profile across the channel is determined by the relationshipbetween the optical phase and the dispersion as defined in Equation 6.The adjacent region 68, which is selected to operate on the nextadjacent wavelength channel (Channel B) in the DWDM signal, has beenseparately configure to only compensate for a predetermined amount ofGVD accumulated by the incident wavelength channel. The linear phaseprofile with respect to frequency across the channel is determined bythe relationship between the optical phase and GCD as defined inEquation 5.

Additionally, amplitude control of the individual optical channels canbe obtained by setting up a phase ramp in the axis orthogonal to thedispersion axis. This type of phase profile can be used to direct apredetermined percentage of each individual channel to a predeterminedoutput port by suitable selection of the phase profile to create adiffractive phase grating that directs the required amount of light intoa particular grating order. This technique can be used as an all opticalreconfigurable ROADM similar to that disclosed in U.S. patentapplication Ser. No. 10/706,901 to Frisken. This phase ramp is typicallyreset each time the effective phase imparted to the light reaches 2π.This method can be used to impart an arbitrary phase shift to the signalsimply by adjusting the position of the reset point.

FIG. 10 shows a three-dimensional representation of an arbitrary phaseprofile set up on a liquid crystal optical wavelength processor for asingle wavelength channel. The height of the individual pixels 43indicates the amount of phase change that it imparted to the lightincident on that pixel. The phase profile in the dispersion axis 57, astaken on line A-A is a quadratic profile 66 as shown in FIG. 11. In theorthogonal plane of the wavelength processor, the phase profile, astaken on line B-B of FIG. 10 is a 2π repeating ramp 67 as shown in FIG.12. Also depicted in FIG. 12 is a second phase ramp 68 indicating how anarbitrary phase shift 69 can be imparted to the light incident on theoptical wavelength processor simply by adjusting the pixel where thereset point occurs.

Note that in the preferred embodiment, the optical signals are notconverted into electronic signals but remain in the optical domain. Thiseliminates the complexity of the system by having separate electronicdevices dedicated to each channel. By keeping the signals in the opticaldomain also increases the capacity of the communications link such thatit is able to with higher bit rate communications signals demanded bynext-generation optical networks. The optical processor itself though iselectronically controllable and so lends itself to activereconfiguration of the compensation as the needs of the opticaltransmission link change from span to span and over time as variouselements in the link degrade.

Compensation of Nonlinear Effects

Optical fibers are capable of supporting a range of nonlinear effectsthat usually result in the generation of new wavelengths. In DWDMcommunications systems the most common nonlinear process that affectsthe OSNR of the optical channels involves the mixing between signals oftwo or more wavelengths through the optical Kerr effect. Nonlinearprocesses can also cause a modification of the phase structure of theoptical pulses propagating in the nonlinear medium. The magnitudes ofthese effects are dependent on the strength of the incident field raisedto some power. The SPM effect is another example of a nonlinear processbut, as described earlier, does not usually affect the integrity of theONSR of the signal since it is an interaction of a pulse with itself.

Nonlinear processes are typically weak, however, in optical fibers thesmall mode-field cross-section in SMF results in high field strengths,even if the total power carried by the fiber is relatively small. Forthis reason, optical nonlinear processes must be seriously consideredand understood when designing an optical fiber communications link.

The most problematic nonlinear process in optical fibers is four-wavemixing (FWM) which is a third-order nonlinear process where theinteraction of three fields leads to the generation of a fourth. Ageneral explanation of the process involves light at two differentfrequencies which interact with the bound electrons in the optical fiberto modulate the refractive index at the difference frequency of thesignals. The light is then modulated as it encounters the indexmodulation and is up- or down-shifted by the difference frequencyresulting in the generation of sidebands to the original opticalsignals.

FWM is governed by the phase-matching condition ω₁+ω₂=ω₃+ω₄ which isrelatively easy to satisfy in optical fibers for the degenerate case ofω₁=ω₂. In this case a strong signal at frequency ω₁ interacts with athird signal at a frequency ω₃=ω₁−Δω to creates a fourth signal locatedsymmetrically about ω₁ at a frequency ω₄=ω₁+Δω. In DWDM systemscontaining multiple channels each separated by a frequency of Δω, theFWM process is effectively seeded at each frequency by the adjacentoptical channels. That is, the n-th optical channel at frequency ω_(n)interacts with itself and with the adjacent channel ω_((n−1)) which isspaced from ω_(n) by Δω. This creates a new signal at frequency ω_(n)+Δωby the FWM process which occurs at a frequency of ω_(n)+Δω=ω_((n+1)).That is, the FWM-product manifests itself as a noise term at thefrequency of the next adjacent channel which degrades the OSNR of theoptical signal at that that frequency. The newly generated signalscontain the phase information of the pump wavelengths which also makesthis process an important cross-talk mechanism in DWDM communicationssystems, particularly those that rely on the phase information of thetransmitted pulse.

FWM is the main nonlinear effect that contributes to the degradation ofthe optical signal. The new frequencies generated by the process causecross terms between two wavelength channels, which in turn degrades theOSNR of each wavelength channel. Cross-phase modulation (XPM) iseffectively a by-product of FWM within the fiber. Another way to thinkabout XPM it that it occurs between two pulses traveling in the fiber offrequencies ω₁ and ω₂. An optical beat is generated by the two pulses atthe difference frequency (ω₂−ω₁). This beat modulates the pulse andgenerates by the FWM process a noise term at ω₃=2ω₂+ω₁. It is this noiseterm at ω₃ that degrades the OSNR of the next adjacent wavelengthchannel since it occurs at the same frequency. FWM interactions arestrongest between two adjacent wavelength channels since the differencein frequency Δω is small which results in a long coherence length inwhich the channels can interact. The coherence length L_(COH) is definedas:

$\begin{matrix}{L_{COH} = {\frac{2\pi}{{\Delta \; \varphi}} \propto \frac{1}{\Delta \; \omega^{2}}}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

where significant FWM occurs for lengths of fiber in the regionL<L_(COH) and can effectively be thought of as the distance that thechannels remain in-phase as they propagate through the fiber. The FWMinteractions are thus strongest between two adjacent wavelength channelssince the difference in frequency Δω is small.

A less significant, but still important, feature intertwined with theFWM effect is the GVD of pulses of different frequencies which propagateat different speeds through the fiber. The GVD difference between twochannels of interest is known as the GVD mismatch. This feature leads toa walk-off effect in the description of nonlinear phenomena involvingtwo or more pulses which overlap in the time domain. More specifically,the nonlinear interaction between two optical pulses ceases to occuronce the faster moving pulse has completely walked through the slowermoving one. The separation between the two pulses is described by awalk-off parameter d₁₂ defined by

$\begin{matrix}{d_{12} = {\frac{1}{v_{g}\left( \omega_{1} \right)} - \frac{1}{v_{g}\left( \omega_{2} \right)}}} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$

where v_(g)(ω₁) is the group velocity of the optical pulse of frequencyω₁. For pulses of width T, the walk-off length can be defined as:

$\begin{matrix}{L_{W} = \frac{T}{d_{12}}} & \left( {{Equation}\mspace{14mu} 9} \right)\end{matrix}$

The FWM of optical signals is also dependent on the phase of theinteracting optical signals as seen from the dependence on the form ofthe propagating wave. The effect is strongest when the pulses arein-phase. Thus, two pulses of frequencies ω₁ and ω₂ overlapping in timeat the beginning of a span in the link, where the coherence between thepulses is also greatest, will experience the greatest nonlinearcoupling. The nonlinear effects accumulate as the pulses propagatethrough the fiber since the relative phase of neighboring channels ismaintained over long distances. The effect on the pulses as theypropagate through the fiber is governed by the “ . . . +σ(z)|E|²E . . .” term of the NLSE (Equation 1), where E is form of the propagating waveis defined in Equation 4.

Current methods of compensating for these nonlinear effects are mainlyfocused towards minimization of the effects rather than compensation.This is usually achieved through management of the dispersion map andthe individual fiber lengths that make up the transmission link. Commondispersion managed links benefit slightly by the GVD mismatch thatoccurs between adjacent channels since the pulses at the beginning ofthe next span are not completely overlapping, however, there is still aresidual amount of coupling that acts to degrade the OSNR across theentire length of the link.

A better solution is to manipulate the optical signals in such a waythat the optical pulses propagating in the link self-compensate for theaccumulated nonlinear build-up. This can be accomplished by manipulatingthe phase relationship of the interacting optical signals. Two opticalsignals that are in-phase interact to generate additional signalsthrough the FWM interaction. The distance over which this interactionoccurs is governed by the walk-off distance L_(w) defined in Equation 9.That is, the nonlinear interaction is strongest in a positive sense whenthe pulses involved are completely over-lapping and in-phase, and theinteraction strength decreases as the overlap decreases and the pulsesundergo a de-phasing with respect to each other as a result of normalpropagation.

The FWM process possesses a reciprocal nature in that, when the twointeracting pulses are completely out-of-phase, the interaction isequally strong but in a negative sense. This can be seen more clearly inFIGS. 13 and 14. FIG. 13 shows a graph of the nonlinear interactionstrength as a function of propagation distance z along one span of SMFin the communications link. Also depicted in this Figure are pictorialrepresentations of two arbitrary signal pulses 50 and 51 of differentfrequencies at various distances along the span. At the start of thefiber (z=0) the two pulses are completely overlapping and are in-phase(depicted by arrows 52 and 53) such that the nonlinear interactionstrength is a maximum in a positive sense. As the pulses propagate alongthe fiber, the overlap between the two pulses decreases due to thedifference in the group velocity such that the interaction strengthdecreases to zero (54) when the pulses have completely walked apart.

FIG. 14 shows a similar graph to that of FIG. 13, however, it depictsthe interaction between two different optical pulses 55 and 56, again ofdifferent frequency, however in this case the relative phase of the twopulses is 180° or π radians i.e. the pulses are out-of-phase. In thiscase, the nonlinear interaction is again initially at a maximum when thepulses are overlapping at the beginning of the span, however, the signof the interaction is negative. Again, as the pulses walk apart due toGVD, the interaction strength decreases to zero as shown at point 57.

This reciprocality in the nonlinear interaction can be exploited is seenin FIG. 15. This Figure is a graph depicting the intensity of the FWMproducts between two optical pulses over two spans of an optical fiberlink. An amplifier/compensation node is included at the end of each span(Node 1 and Node 2). At the start of the link (z=z₁) two pulses 70 and71 of different frequencies are overlapping in in-phase. The nonlinearinteraction strength is thus at a maximum between the pulses and theintensity of the FWM product is increasing at a rapid rate. As thepulses propagate along the fiber in the z-direction they walk apart(z=z₂) so that the interaction strength decreases and the rate theintensity of the FWM product builds up decreases. When the pulses havecompletely walked apart (z=z₃), the intensity of the FWM product hadreached a maximum level and does not increase any further along thespan.

At the first amplifier/compensation node (Node 1) there is installed anEDFA to compensate for signal amplitude distortion and an opticalwavelength processor as described previously. The optical processorcompensates the GVD and dispersion that the pulses have experienced onpropagation through the first span which bring the two pulses back totheir initial relative relationship in time. In this case the two pulsesare brought back into an overlapping relationship where the magnitude ofthe nonlinear interaction is a maximum (at point z=z₄). Additionally,the optical processor also imparts an additional phase shift ofπ-radians to one pulse relative to the other (in a similar fashion tothat described earlier and depicted between Channels A and B of FIG. 2).This ensures that the two pulses are now completely out-of-phase suchthat the sign of the nonlinear interaction is negative. Now, as thepulses propagate, the FWM products which were generated in the firstspan are effectively unwound due to the reciprocal nature of the FWMprocess. Thus, as can be seen in FIG. 15, as the pulses propagatethrough Span 2 of the optical link the intensity of the FWM productsinitially generated in Span 1 decreases with a rate proportional to theamount of overlap between the pulses in other words, the nonlinearity isunwound such that, at the end of the second span, the intensity of theFWM products of the two pulses is substantially zero again.

As mentioned previously, typical prior art dispersion managed opticalcommunications links are most effective when the optical signals travelthe entire length of the managed link so that they attain the fullbenefit of the compensating stages. In reconfigurable optical linkswhere signals are being added and drop at various points along the link,the signals are not receiving the benefit of the compensating stages andso the accumulated nonlinear effects can be quite varied. A method ofimproving this situation uses the active reconfigurable nature of thepreferred embodiments of the current invention to minimize thisnonlinear build-up. By using the above-described method where thenonlinearities are unwound between adjacent channels over two spans, theworst-case scenario of a signal is added to the link with respect to theaccumulated nonlinearity would be that acquired over only two spans.

In one technique, every pair of two adjacent channels (for examplechannels 1 and 2, 3 and 4, 5 and 6 etc) can be treated in an isolatedfashion so that the FWM products between the two channels do not buildup. That is, at Node 1 after the first span (and every second node afterthat) the CD in the channels is completely compensated and the GVDmismatch between the pairs of adjacent channel is returned tosubstantially zero. Every second channel (i.e. channels 2, 4, 6, etc)also receives an additional phase shift of □ radians to put the channelout of phase with its adjacent channel. This allows the nonlinearitiesbetween the channels to unwind during propagation through the next spanin a similar fashion to that described in FIG. 15.

This technique is effective for this first set of channel pairs (i.e.channels 1 and 2, 3 and 4 etc.), however, it does not take into accountthe nonlinear FWM effects that accumulate between channels 2 and 3 (and4 and 5, 6 and 7 etc). A technique of minimizing this is to perform asimilar operation at each subsequent node i.e. Nodes 2, 4, 6, etc.,however, instead of compensating the GVD of channels 1 and 2, therelative GVD between channels 2 and 3 is substantially returned to zeroat these nodes.

Monitoring of the GVD can be advantageous for periodic polling of thesystem to check the calibration factors as the system ages and isexposed to temperature variation which cause drift in the properties ofthe optical fiber components. This may be achieved by extracting theclock signal from each of the wavelength channels and comparing thephase of the received clock with its previous values and against theclock signals of the other channels.

This method is illustrated more clearly in FIG. 16 which shows a graphof the GVD for four optical channels ω₁ to ω₄ (91 to 94 respectively).The shaded regions 95 indicate the region along each span where the FWMnonlinear interaction is strongest. Each of the channels is initiallyconsidered to have a relative GVD of zero (i.e. ΔT=0) and no CD.

Each of the channels experiences CD and GVD in the SMF of the span thatis partially compensated by the subsequent length of DCF in the normalfashion. At the end of Span 1 (96) the channels have retained a certainamount of CD (depicted by the width of the channel in the vertical axisof the graph) and a GVD mismatch. After the amplification stage in thefirst Node (not indicated) the signals are fed into the preferredembodiment of the optical processor of the current invention. The CD isfirst compensated (indicated by region 97) and the clock signals GVDbetween the pairs of channels (ω₁ and ω₂) and (ω₃ and ω₄) is returned toits initial value of ΔT=ΔT₁₂=ΔT₃₄=0 at the end of Node 1 (98). Inaddition to the GVD correction between adjacent pairs of channels, eachsecond channel (i.e. Channels 2 and 4) each receive a phase shift ofπ-radians so that the nonlinear effects accumulated in Span 1 areunwound during propagation through Span 2 since the relative GVD betweenthe channel pairs in Span 2 remains substantially equivalent to therelative GVD during propagation through Span 1. The GVD between channels2 and 3 (ΔT₂₃) which at this point is equal to twice the residual GVD isnot adjusted at this stage.

At Node 2 the CD of each of the channels is again compensated (depictedby region 99), however, in this case the GVD mismatch between Channels 2and 3 is adjusted to be equal to it's value at Node 1. One of thesechannels (say Channel 3) also receives an additional phase shift ofπ-radians so that the nonlinear effects that have accumulated betweenthese two channels during propagation through Spans 1 and 2 begins tounwind as the channels propagate through the next span.

At each node in the transmission link, the CD and GVD can be activelyreconfigured to take account of the particular fibers and thetransmission lengths of the previous span. Initial testing with a firstgeneration model of the preferred embodiment has been shown to providegreater than ±100 ps/nm across a channel of about 60 GHz in width. For achannel width of 30 GHz, the amount of CD compensation has been shown tobe greater the 500 ps/nm. In conjunction with existing passive CDcompensators, this amount of adjustable compensation is more thansufficient to cope with next generation optical networks operating at 40Gbit/s. These limits have been determined for the current embodiment andare a result of choices made for the amount of dispersion compensationand the numerical aperture of the beam. Further optimization of thesystem parameters is possible to improve the amount of achievablecompensation.

The independent control of phase and intensity at each network nodeoffered by the preferred embodiments of the current invention opens up avast array of potential system-level applications such as:

CD compensation, trimming and tailoring on a per-channel basis.

Suppression of nonlinear transmission effects such as FWM, group delay,and phase compensation.

Custom tailored optical filtering to maximize system OSNR.

Phase to amplification conversion for example as a phase-shift-keyed(PSK) receiver.

-   -   Seamless adding and dropping of wavelength channels including        reconfiguration of the network architecture and multiple        transmission standards (for example 10 Gbit/s and 40 Gbit/s)        within a single device.

The optical communications system and methods described herein, and/orshown in the drawings, are presented by way of example only and are notlimiting as to the scope of the invention. Unless otherwise specificallystated, individual aspects and components of the optical communicationssystem and methods may be modified, or may have been substitutedtherefore known equivalents, or as yet unknown substitutes such as maybe developed in the future or such as may be found to be acceptablesubstitutes in the future. The optical communications system and methodsmay also be modified for a variety of applications while remainingwithin the scope and spirit of the claimed invention, since the range ofpotential applications is great, and since it is intended that thepresent optical communications system and methods be adaptable to manysuch variations.

It will be appreciated that the methods described above at leastsubstantially provide an improved method of compensating for opticalsignal degradation of signals propagating in optical fibers in DWDMtelecommunications applications on a per-channel basis.

1-24. (canceled)
 25. In an optical transmission system including: anoptical transmitter for transmitting a plurality of independentwavelength channels; at least first and second spans, each said spanrespectively including: at least a first length of transmission opticalfibre characterized by a positive dispersion coefficient; and at least afirst length of dispersion-compensating optical fibre disposed aftersaid length of transmission fibre and characterized by a negativedispersion coefficient for substantially compensating a large proportionof the group velocity delay accumulated by said wavelength channels insaid first span; a first node module disposed after said first spanincluding: an optical amplifying module; and a phase manipulation modulefor: a. substantially compensating for dispersion of said wavelengthchannels accumulated in said first span; b. substantially compensatingfor the remaining accumulated group delay in selected pairs of first andsecond wavelength channels; and c. modifying the phase of said secondwavelength channel in said selected pair such that the cross talkbetween said selected pair of channels generated in the first link is atleast partially compensated in the second link.
 26. A system as claimedin claim 25 further including a second node module disposed after saidsecond span including; an optical amplifying module; and a phasemanipulation module for substantially compensating for the remainingaccumulated group delay in selected pairs of third and fourth wavelengthchannels and for modifying the relative phase of said wavelengthchannels.
 27. A system as claimed in claim 26 including: a plurality ofsequentially disposed pairs of said first and second spans, each saidfirst and second span interspersed between one of a plurality ofrespective first and second node modules; and a receiver module forreceiving the transmitted wavelength channels.
 28. In an opticaltransmission system including: an optical transmitter for transmitting aplurality of independent wavelength channels; at least first and secondlengths of transmission optical fibre characterized by a firstpredetermined dispersion coefficient; at least first and second lengthsof dispersion-compensating optical fibre characterized by a secondpredetermined dispersion coefficient for substantially compensating alarge proportion of the group velocity delay accumulated by saidwavelength channels in said respective lengths of transmission opticalfibre, said first length of dispersion compensating fibre disposed aftersaid first length of transmission fibre and said second length ofdispersion compensating fibre disposed after said second length oftransmission fibre; a first node module interposed between said firstlength of dispersion compensating fibre and said second length oftransmission fibre including: a phase manipulation module configured to:substantially compensate the chromatic dispersion accumulated by saidwavelength channels in said first length of transmission fiber;substantially compensate for the remaining accumulated group delay inselected pairs of first and second wavelength channels; modifying thephase of said second wavelength channel in said selected pair such thatthe modified phase of said second channel is substantially out of phasewith said first wavelength channel in said selected pair; and a secondnode module disposed after said second length of dispersion compensatingfibre including; an optical amplifying module; and a phase manipulationmodule for substantially compensating for the remaining accumulatedgroup delay in selected pairs of third and fourth wavelength channelsand for modifying the phase of said wavelength channels such that themodified phase of all said wavelength channels are substantiallyin-phase.
 29. A system as claimed in claim 28 wherein said phasemanipulation module modifies the phase of said wavelength channels in adecoupled manner such that nonlinear optical effects accumulated by anyof said wavelength channels in said first length of transmission fibreare reversed in said second length of transmission fibre.
 30. A systemas claimed in claim 28 wherein said nonlinear optical effects includemulti-channel mixing effects.
 31. A system as claimed in claim 28wherein said first and second predetermined dispersion coefficients areof opposite sign.